It is well known that rail vehicles travel in a guided manner. The forces required for guidance are produced in the area of contact between wheel and rail, the wheel/rail contact. However, these forces are also responsible for negative effects on the rails and wheels. For example, tangential forces, which are always associated with sliding effects and therefore with friction, cause profile wear due to material abrasion. In addition, at sufficiently high levels, the forces acting on wheel and rail stress the material, resulting in rolling contact fatigue (RCF). This produces e.g. hairline cracks in the rail and/or wheel. A typical form of rail surface damage caused by RCF are head checks. In the wheel, cracks may occur below the surface, propagate outwards and lead to significant flaking. However, the cracks can also occur on the surface, propagate inwards and likewise result in material break-outs, as occurs e.g. with the well-known herringbone pattern phenomenon. In the case of surface initiated cracking, the effect occurs that the incipient cracks are partially removed again by the above mentioned profile wear, which means that a certain degree of profile wear may in some cases be desirable. In addition to the tread damage referred to above, a number of other types of damage such as e.g. wheel flats, material deposition, transverse cracks in the wheel tread, etc. also occur.
Wheel/rail contact therefore assumes particular safety-relevant importance also in the case of high-speed trains, for example. Wheel/rail contact irregularities caused e.g. by severe damage to a wheel may result in consequential damage or even derailment. However, even minor damage such as hairline cracks can cause major problems, as repairs will be required resulting in high costs and possible train service delays.
A number of mechanical devices for guiding a rail vehicle are therefore known. Many of the known systems are based on optimizing the radial position of the wheels in the track when negotiating curves in order to reduce the forces acting on the independently rotating wheelsets or conventional wheelsets of a wheel truck or vehicle, thereby reducing, so the argument goes, the friction and therefore the profile wear in the wheel/rail contact.
For example, EP 0 600 172 A1 describes a wheel truck for rail vehicles wherein the wheelsets are turned out with respect to the truck frame by means of force-controlled final control elements when negotiating curves. Here, however, no radial position of the wheelsets relative to the track is implemented, but only the angle between wheelset and truck frame is adjusted according to the radial position. Although this provides favorable wear behavior in many operating conditions, this is less than optimum.
DE 44 13 805 A1 discloses a self-steering three-axle wheel truck for a rail vehicle in which the two outer wheelsets are provided with a radial controller and the inner wheelset can be moved transversely to the direction of travel by an active final control element. This reduces the lateral forces on the outer wheelsets—when the active final control element is suitably acted upon, a third of the centrifugal force is exerted on each wheelset. This means that all three wheelsets are used for control when negotiating curves and the orientation of the wheelsets relative to the center of the curve is improved.
Another method of this kind may be found in EP 1 609 691 A1 of the Applicant.
The common feature of all these methods is that they aim to minimize wheel/rail contact friction and therefore profile wear. In these methods, the position of the wheels relative to the track is influenced such that sliding effects at the point of contact are prevented or minimized. However, rolling contact fatigue also results in rail and wheel damage. To rectify this damage, a degree of friction is quite desirable, as cracks produced in the material can be surface abraded thereby. Minimum friction does not therefore always correspond to an optimum rail/wheel loading ratio.